The present invention relates to a Coriolis flowmeter.
Coriolis flowmeters are commonly used devices for measuring a flow of a fluid through a pipe. In various branches of industry, for example in the chemical industry, in the food industry or in the pharmaceutical industry, the measurement data obtained by Coriolis flowmeters is used to control complex industrial processes.
Coriolis flowmeters operate on the Coriolis Effect. A mass flow dependent Coriolis force occurs when a moving mass is subjected to an oscillation perpendicular to the flow direction. Coriolis flowmeters generally comprise at least one measurement tube and a driver for setting the tube into an oscillatory motion. In operation, the fluid flows through the oscillating tube. One type of driving mechanism is an electromechanical driver that imparts a force proportional to an applied excitation signal, i.e. a current or a voltage. Measurement tube and fluid form an oscillatory system that is normally operated at a resonance frequency. The resonance frequency depends on the material of the measurement tube and a density of the fluid. The Coriolis force is induced by the oscillatory motion. A Coriolis reaction force experienced by the traveling fluid mass is transferred to the measurement tube itself and is manifested In a change of motion of the tube. Two motion sensors detect this change. Mass flow is usually determined based on a phase difference between the measurement signals derived by the motion sensors.
In order to operate the flowmeter in a resonant mode of vibration, most flowmeters comprise a feed back loop generating the excitation signal based on the measurement signals of the motion sensors. Feeding back the sensor signal to the driver permits the drive frequency to migrate to the resonant frequency.
Coriolis flowmeters are used in applications where repeatable and stable measurement of liquid mass or volume flow is required. In applications where successive batches of products are processed conventional flowmeters can suffer from poor meter accuracy and repeatability. The reason for this is that whenever a sudden transition from a full measurement tube to an empty measurement tube or vice versa occurs, the physical properties of the oscillatory system change dramatically and the flow meter needs time to adjust in order to establish a desired mode, frequency and amplitude of the motion for the respective measurement situation. During this transition time, conventional coriolis flow measurement based on the measurement signals derived by the motion sensors do not produce accurate results. Erroneous flow spiking or outputs during no-flow situations are common operational problems.
Currently, mass flow meter manufacturers approach solutions to overcome this problem in four ways:                1) The meter exceeds a manufacturer predefined limit of excitation signal or drive gain and faults, locking the output variables at a fail-safe condition.        2) The meter has two separate drive circuits, evaluates the drive gain from normal conditions, and compensates for this change, while applying a mathematical algorithm adjusting the measured mass flow variable.        3) A density limit is programmed into the meter to account for the detected change in the measured product density, as a means to regulate a fail-safe point.        4) A low flow cut-off is elevated.        
Unfortunately, these four methods result in measurement accuracy and performance issues:                A) The predefined fault limit may be significantly higher than the actual process condition which produces an erroneous flow output or the meter output locks up, requiring frequent manual intervention.        B) Process evaluation of two drive circuit gain levels can be falsely interpreted and the resulting measuring uncertainty exceeds 2%.        C) The density limit point or range is slower to react to the change, including reaction to empty measurement tube conditions.        D) Elevation of the low flow cut-off can miss measurable product flow or is ineffective as a recognition technique.        
Some of these approaches are for example described in U.S. Pat. No. 6,505,519 B2. In this patent, a self-validating meter is described, which provides a best estimate of a parameter to be measured, e.g. mass flow, based on all information available. This flowmeter comprises a controller, which derives a raw measurement value based on the sensor signals of the motion sensors. When the controller detects no abnormalities, the controller has nominal confidence in the raw measurement value and produces a validated measurement value equal to the raw measurement value. When the controller detects an abnormality in the sensor, it produces a validated measurement value, which is a value that the controller considers to be a better estimate of the parameter to be measured.
It is an object of the invention to provide a Coriolis flowmeter, with improved measurement accuracy, especially in measurement applications, where conventional Coriolis flowmeter may have a reduced accuracy due to erroneous flow spiking or outputs during no-flow situations.
To this end, the invention comprises a Coriolis flowmeter comprising:                a measurement tube,        a driver for imparting a force proportional to an applied excitation signal to the measurement tube for setting the measurement tube into an oscillatory motion,        motion sensors for measuring the motion of the measurement tube,        an excitation signal generator, for generating the excitation signal to be supplied to the driver based on the measurement signals derived by the motion sensors,        means for monitoring the excitation signal and for determining whether an amplitude of the excitation signal exceeds an application-specific range, and        means for determining a corrected flow of the fluid through the measurement tube,        wherein the corrected flow is determined based on measurement signals derived by the motion sensors, when the excitation signal has an amplitude, that lies within the application-specific range and        wherein the corrected flow is set to an application-specific flow, when the excitation signal has an amplitude that exceeds the application-specific range.        
According to a refinement of the invention, the application-specific range for the amplitude of the excitation signal corresponds to a range for the amplitude that will occur under normal operation conditions of the specific application, wherein a maximal amplitude occurs, when the flow reaches a maximal flow to be expected for the specific application and a minimal amplitude occurs, when the flow is zero.
According to a further refinement, the flowmeter comprises a separate output, solely for providing an output signal representing the corrected flow.
According to another refinement, the flowmeter comprises a relay connected to an output representing the preliminary flow, which will set the output representing the preliminary flow to the application-specific flow, when the excitation signals exceeds the application-specific range.
Further, the invention comprises a method of determining the application-specific range of the amplitude of the excitation signal for a flowmeter according to the invention, including the steps of                running one or more test cycles, by sending consecutive batches of product through the flowmeter,        monitoring the amplitude of the excitation signal during the test cycles,        determining the minimal and the maximal amplitude of the excitation signal during periods of normal operation of the flowmeter during the test cycles, and        setting the application-specific range accordingly.        
According to a refinement of the method, the maximal amplitude of the excitation signal during normal operating conditions is based on a test cycle run with a product having the highest density and/or highest viscosity of the products to be used in the specific application.
Further, the invention comprises a Coriolis flowmeter comprising:                a measurement tube,        a driver for imparting a force proportional to an applied excitation signal to the measurement tube for setting the measurement tube into an oscillatory motion,        motion sensors for measuring the motion of the measurement tube,        an excitation signal generator, for generating the excitation signal to be supplied to the driver based on the measurement signals derived by the motion sensors,        means for deriving and monitoring a damping coefficient of the motion of the measurement tube and for determining whether the damping exceeds an application-specific range, and        means for determining a corrected flow of a fluid through the measurement tube,        wherein the corrected flow is determined based on measurement signals derived by the motion sensors, when the damping coefficient lies within the application-specific range, and        wherein the corrected flow is set to an application-specific flow, when the damping coefficient exceeds the application-specific range.        
The invention and further advantages are explained in more detail using the figures of the drawing, in which four exemplary embodiments are shown.